Electrophotographic (EP) printers deposit toner on a receiver in a pattern defined by a charge pattern (“latent image”) on a photosensitive member (“photoreceptor”). Photoreceptors are generally drums or belts that rotate to produce successive images on one or more receivers. Any given point on the photoreceptor, as it rotates 360°, passes through a sequence of steps that make up the EP process.
When toner is transferred from the photoreceptor to the receiver, it is not always transferred completely. To maintain image quality, it is desirable to remove this residual toner before applying a new toner pattern to the photoreceptor. In general, before charging the photoreceptor at the beginning of the electrophotographic image-forming process, it is desirable to prepare the photoreceptor for charging. Cleaning toner off the photoreceptor is a mechanical reconditioning process. The mechanical reconditioning is part of the photoreceptor-preparation process that prepares the photoreceptor for its next rotation (or its first rotation, e.g., when run at printer startup). Photoreceptor preparation also includes electrical reconditioning: the latent image is erased to prevent residual image formation. Residual image formation, also called “ghosting” or “ghost-image formation,” is the appearance in a subsequently-printed image of part of a previously-printed image, and undesirably reduces image quality. Various schemes have been developed for performing this photoreceptor preparation process.
U.S. Pat. No. 7,286,788 to Kinoshita et al. describes two auxiliary charging brushes in contact with the photoreceptor between the transfer zone, in which area toner is transferred from the photoreceptor to the receiver, and the charger, which uniformly charges the photoreceptor to prepare it to receive a new latent image. The first brush has an AC and a DC bias to recover residual toner and erase the latent image on the photoreceptor. The second brush has a DC bias to charge any remaining residual toner so that it does not transfer to the charger.
However, in this scheme some toner remains on the photoreceptor on successive revolutions of the photoreceptor. This toner can be transferred to the receiver and appear as noise in an image or as an outline of a previous image. Either reduces image quality. Moreover, the mechanical contact of the two brushes with the photoreceptor produces friction. This friction increases the power required to drive the photoreceptor, and can produce mechanical wear on the surface of the photoreceptor that will eventually render the photoreceptor unsuitable for use.
U.S. Pat. No. 6,163,672 to Parker et al. describes using tri-level xerographic modules to create images having custom charged-area development (CAD) and discharged-area development (DAD) image areas. However, this scheme does not provide mechanical reconditioning (e.g., toner removal) after re-charging and erasing. Instead, additional toning is performed.
It is known to use detack corona chargers to help release receiver sheets from transfer rollers. In various schemes, detack corona chargers are designed to spray enough charge to reduce the electrostatic force of attraction between the receiver and the photoreceptor but not so much as to degrade transfer efficiency. However, if this scheme were used to try to release toner from a photoreceptor by charging all or substantially all the toner on the photoreceptor to the same sign, trapped charges created after exposure to an erase lamp could remain in the photoreceptor, possibly reducing image quality by producing ghost images. There is, therefore, a continuing need for an improved way of effectively preparing a photoreceptor without introducing mechanical wear and without causing image artifacts.